UMaryland: Prediction of Modified Gravity Theory Confirmed

[bystander]

Gas Rich Galaxies Confirm Prediction of Modified Gravity Theory
University of Maryland | 2011 Feb 22

Recent data for gas rich galaxies precisely match predictions of a modified theory of gravity know as MOND according to a new analysis by University of Maryland Astronomy Professor Stacy McGaugh. This -- the latest of several successful MOND predictions -- raises new questions about accuracy of the reigning cosmological model of the universe, writes McGaugh in a paper to be published in March in Physical Review Letters.

Star dominated spiral galaxy UGC 2885
Image by Zagursky & McGaugh

Modern cosmology says that for the universe to behave as it does, the mass-energy of the universe must be dominated by dark matter and dark energy. However, direct evidence for the existence of these invisible components remains lacking. An alternate, though unpopular, possibility is that the current theory of gravity does not suffice to describe the dynamics of cosmic systems.

A few theories that would modify our understanding of gravity have been proposed. One of these is Modified Newtonian Dynamics (MOND), which was hypothesized in 1983 by Moti Milgrom a physicist at the Weizmann Institute of Science in Rehovot, Israel. One of MOND's predictions specifies the relative relationship between the mass of any galaxy and its flat rotation velocity. However, uncertainties in the estimates of masses of stars in star-dominated spiral galaxies (such as our own Milky Way) previously had precluded a definitive test.

To avoid this problem, McGaugh examined gas rich galaxies, which have relatively fewer stars and a preponderance of mass in the form of interstellar gas. "We understand the physics of the absorption and release of energy by atoms in the interstellar gas, such that counting photons is LIKE counting atoms. This gives us an accurate estimate of the mass of such galaxies," McGaugh said.

Using recently published work that he and other scientists had done to determine both the mass and flat rotation velocity of many gas rich galaxies, McGaugh compiled a sample of 47 of these and compared each galaxy's mass AND rotation velocity with the relationship expected by MOND. All 47 galaxies fell on or very close to the MOND prediction. No dark matter model performed as well.

"I find it remarkable that the prediction made by Milgrom over a quarter century ago performs so well in matching these findings for gas rich galaxies," McGaugh said. "

MOND vs. Dark Matter - Dark Energy

Gas rich LSB galaxy, F549-1
Image by Zagursky & McGaugh

Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by dark matter - dark energy theory. However, according to McGaugh this cosmology does not account well for what happens at the scales of galaxies and smaller.

"MOND is just the opposite," he said. "It accounts well for the 'small' scale of individual galaxies, but MOND doesn't tell you much about the larger universe.

Of course, McGaugh said, one can start from the assumption of dark matter and adjust its models for smaller scales until it fits the current finding. "This is not as impressive as making a prediction ahead of [new findings], especially since we can't see dark matter. We can make any adjustment we need." This is rather like fitting planetary orbits with epicycles," he said. Epicycles were erroneously used by the ancient Greek scientist Ptolemy to explain observed planetary motions within the context of a theory for the universe that placed the earth in its center.

"If we're right about dark matter, why does MOND work at all?" asks McGaugh. "Ultimately, the correct theory - be it dark matter or a modification of gravity - needs to explain this."

A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies - SS McGaugh

• arXiv.org > astro-ph > arXiv:1102.3913 > 18 Feb 2011

[bystander]

More Evidence Against Dark Matter?
Science NOW | Adrian Cho | 2011 Feb 25

Thousands of physicists, astrophysicists, and astronomers are searching for dark matter, mysterious stuff whose gravity seems to hold the galaxies together. However, an old and highly controversial theory that simply changes the law of gravity can explain a key property of galaxies better than the standard dark-matter theory, one astronomer reports. That claim isn't likely to win over many skeptics, but even some theorists who favor the standard theory say the analysis hands them a homework problem they should solve.

"The standard theory should explain this, and it doesn't yet. That's fair to say," says Simon White, a cosmologist at the Max Planck Institute for Astrophysics in Garching, Germany, who was not involved in the current analysis.

In 1933, Swiss astronomer Fritz Zwicky suggested the existence of dark matter when he found that the galaxies in a particular cluster swirl about each other too fast to be bound by their gravity alone. In the 1970s, American astronomer Vera Rubin and others discovered that the stars at the edges of individual galaxies also appear to move too fast to be held by the gravity of the stars in the center. Those outer stars ought to move more slowly than the ones circling closer in—just as Jupiter orbits the sun more slowly than Earth. Instead, the speed of the stars generally increases with the distance from the galactic center, eventually flattening out at a maximum value. That observation seemed to clinch the case for some sort of dark matter.

Or did it? In 1983, Mordehai Milgrom a physicist at the Weizmann Institute of Science in Rehovot, Israel, found that he could explain the so-called galaxy rotation curves without dark matter if he simply assumed that on the galactic scale, dynamics and gravity worked a bit differently from what Isaac Newton postulated. Specifically, Milgrom assumed that for very small accelerations, the square of the acceleration, not just the acceleration, is proportional to the gravitational force.

For the past 28 years, Milgrom's idea, known as Modified Newtonian Dynamics (MOND) has generated a long-simmering debate. Many researchers argue that ever more evidence from clusters of galaxies, the largest scale structure of the universe, and the afterglow of the big bang points to the existence of dark matter. Still, a few researchers counter that when they look at the details, MOND does a better job—at least on the galactic scale.

Now, in the latest shot from the MOND side, Stacy McGaugh, an astronomer at the University of Maryland, College Park, reports that MOND can explain an observed correlation between the mass and the rotation speed of galaxies—that is, the speed of those outer stars—called the baryonic Tully-Fisher relation. MOND researchers had tried to do this before, but for their models to work, they had to make an untested assumption about the relationship between a star's mass and the amount of light it puts out. That assumption introduces a large uncertainty, weakening the argument.

To avoid that problem, McGaugh gathered data from various sources on 47 galaxies that contain more hydrogen gas than stars. The mass of the gas can then be estimated directly. McGaugh made a plot of visible mass versus rotation speed for the galaxies. He then plotted the prediction that comes straight out of MOND in a few lines of algebra. The MOND line went right through the data. "You draw the line and the data fall right on it," McGaugh says. "No muss, no fuss." He reports the result in a paper in press at Physical Review Letters.

Crucially, McGaugh finds very little scatter in the data—just what would be expected if the mass of gas and stars was directly determining the rotation speed. It's not clear exactly what dark-matter models would predict, McGaugh says. However, such models make no strong connection between the amount of visible matter and the rotation speed. Indeed, galaxies with the same mass of dark matter can have different numbers of stars. So it would be surprising if dark-matter models yielded such a tight correlation.

"I think the data are good, and the fact that MOND fits is striking," says White, who has worked extensively on simulating the evolution of the universe. "I think Stacy is right in holding this up and saying [to dark-matter modelers], 'Look at this [correlation]. Go see if you can explain it.' " Still, White says, dark matter can explain the variations in the afterglow of the big bang and other cosmological data with which MOND struggles.

But whether MOND is right may be beside the point, says Jerry Sellwood, a theoretical astrophysicist at Rutgers University in New Brunswick, New Jersey. "The real strength of Stacy's paper is that it points to something that can't be explained in cold dark matter, irrespective of whether MOND is right." At the least, Sellwood says, McGaugh deserves credit for keeping others honest about what their models can do.